Published: October 29, 2009 09:41 pm      

Here’s what to look for in a new telescope

Our Planetarium program in November is “Sky Exploring with Telescopes” shown today, Nov. 8, Nov. 15 and Nov. 29 at 4 p.m. and 7 p.m. This column is meant to complement our Planetarium program where visitors can see a number of different types of small telescopes on display.

If you already have a telescope that works (or doesn’t work) or are considering acquiring a telescope, I hope you will find some the below information useful. When most people consider a telescope, they think of a long hand held spyglass, shown in many old movies set on sailing ships.

This kind of telescope uses lenses to bend light and form an image, usually at magnifications from 10 to 20 power. So objects viewed through the telescope appear 10 to 20 times closer. A ship 2 miles away might appear only 0.2 to 0.1 miles away, close enough to recognize the type of flag displayed by the ship.

You can hand hold such a telescope if you lean against part of the ship to reduce your own trembling, which is exaggerated by the telescope’s magnification. Then there is the motion of the entire ship which you have to compensate for.

When using such a telescope for gazing at something up in the sky, it is even harder to find than a ship along the horizon. (When something is near the sky-water horizon, you learn quickly if you are too high (see the sky) or too low (see the water). Then you can just sweep along the horizon to find the ship.

So for a spyglass or lens telescope, a good tripod to hold the telescope is essential for a crisp view of sky bodies. Some small telescopes come with a mounting ring that allows you to attach the telescope directly to a camera tripod which has a one quarter-20 bolt on the top for a standard camera.

For other telescopes, you may have to make your own mounting cradle that will grab the telescope (with a hose clamp) so it is fastened to a block of wood. On the bottom side of this block of wood, you need to install a one quarter-20 screw inlet to allow attachment to a good camera tripod.

Now most lens telescopes sold through catalogs or in discount stores come with a tripod. Often the tripod is the weakest part of these telescopes, not allowing easy pointing of the telescopes at the moon or planets. A heavy duty camera tripod might give you better control and steadier views.

Another weak part of these telescopes are the eyepieces provided. Each eyepiece has printed on it a different number, representing the focal length (focusing distance) in millimeters. (There are 25.4 millimeters in an inch.)

To find the telescope magnification for each eyepiece, you must divide the front lens focal length (its focusing distance) by the eyepiece focal length. (Example, if your front lens has a focal length of 700 mm and the eyepiece focal length is 25 mm, then the magnification = 700 mm/25 mm = 28 power.)

If you find that the stars seen through an eyepiece are “furry” near the edge of the field, a more expensive eyepiece may considerably improve your view. Also be sure that your telescope has a finder scope so that you can view the desired object through the finder scope and find it in the middle of the main telescope field.

While the lens telescopes offer crisp views of the moon and the near planets, a mirror telescope grabs more light and gives you brighter views of star clusters, gas clouds and galaxies.

Most mirror scopes have an open tube at the bottom of which is a concave mirror that focuses light just like a lens. The advantage of the mirror scopes are the lack of false colors in the image. Also the mirrors can be made more easily and less expensively than the same diameter lens telescope. (A typical lens telescope has a lens 75 mm across while a typical mirror telescope of about the same price has a mirror 150 mm or 6 inches wide.)

The stars seen through the 150 mm mirror scope will be 4 times as bright as those seen by the 75 mm lens telescope. (This is because the area of lens or mirror is proportional to the diameter squared; area is what makes images bright.)

The drawback of most mirror scopes is that they must be put out well in advance (20 minutes) of their being used to sky gaze to eliminate air currents circulating in the open tube (due to a difference in temperature between the tube and the outside air.)

If any reader has a telescope that they would like some help with, please bring your telescope in your vehicle to one of the planetarium programs (either at 4 p.m. or 7 p.m.). I will be glad to look at it after the program finishes.

The moon will be full tomorrow evening, rising before sunset (about 5:10 p.m. Standard Time) and setting after the sun rises. The moon tomorrow evening is in Aries the Ram. On Thursday evening, the moon will be near Aldebaran, the star marking the eye of Taurus. This full moon is called the Hunters’ Moon, offering extra evening moonlight for several evenings following. The Hunters’ Moon is similar to last month’s Harvest Moon, which also offered extra moonlight in the early evening.

Bob Doyle can be reached by e-mail at rdoyle@frostburg.edu